Heat-resistant members subjected to repeated heating and cooling include for instance exhaust equipment such as exhaust manifolds, etc. for use in internal engines of automobiles. Exhaust equipment thermally expands by exposure to a high temperature during engine operation and thermally shrinks by cooling by the air when the engine is stopped. This repeated heating and cooling exerts a large thermal load to the exhaust equipment. When the thermal load becomes excessive, the heat-resistant member is subjected to heat deformation and/or heat cracking, causing problems such as gas leaks, etc.
However, since there are many heat-resistant members having extremely complicated shapes, it is generally difficult to predict the generation of heat cracks. Accordingly, test models of heat-resistant members have conventionally been produced from cast iron or cast steel and tested under practical heating conditions, suffering from the problems of extremely long test time and labor.
A main cause of cracks generated in the exhaust equipment repeatedly subjected to high-temperature heating and cooling is that local overheat or heat accumulation (hereinafter referred to as "hot spot") takes place in the exhaust equipment during temperature elevation. Thus, if the positions of the hot spots can be located accurately, it would be possible to predict where cracking may take place. Positions at which large thermal strain is concentrated during temperature elevation correspond to portions in which there is a large plastic compression deformation affecting thermal deformation and heat cracking. Thus, if positions at which thermal strain is concentrated can be located accurately, it would be possible to predict the thermal deformation and heat cracking of the heat-resistant members.
Under such circumstances, the applicant previously proposed a model made of a polyurethane foam similar to a heat-resistant member (Japanese Patent Laid-Open No. 3-28731). In Japanese Patent Laid-Open No. 3-28731, a hot air is introduced into a polyurethane foam model with its inlets and outlets restrained to measure a temperature distribution on a model surface by a thermal image analyzer and also to measure a thermal strain distribution by strain gauges attached to the model surface, thereby predicting portions in which heat cracking may take place.
However, the polyurethane foam model has as small a thermal conductivity as about 1/2500 of those of heat-resistant cast steel or heat-resistant cast iron. Accordingly, when a polyurethane foam is used for a thermal analysis model, it can achieve only an extremely small thermal conductivity. Thus, comparing with the measurement results of temperature distribution of a practical heat-resistant member, accuracy should be improved in the measurement of transitional temperature changes occurring in a short period of time, though the polyurethane foam model may be effectively used in a stationary measurement. Also, since the heat resistance of the polyurethane foam is practically 50.degree. C. or lower, the temperature distribution and strain level on a thermal analysis model surface are easily affected by a temperature variation in a measurement chamber.
Further, to produce a thermal analysis model from a polyurethane foam, it is necessary to read complicated drawings of a heat-resistant member to cut a polyurethane foam block into a three-dimensional model. However, this requires an extremely high skill and a long time which is usually about two or three weeks.
Accordingly, an object of the present invention is to provide a thermal analysis model with which the positions of local hot spots and thermal strain affecting the durability of a heat-resistant member subjected to heating and cooling can be highly accurately located, thereby being capable of predicting the heat cracking of the heat-resistant member.
Another object of the present invention is to provide a method for predicting portions of a heat-resistant member in which heat cracking may take place by means of a three-dimensional thermal analysis model, thereby determining a structure of the heat-resistant member free from heat cracking.